Log periodic antenna with plural crossed dipoles



Nov. 30, 1965 A. KRAVlS ETAL 3,221,332

LOG PERIODIC ANTENNA WITH PLURAL CROSSED DIPOLES Filed April 29, 1960 2 Sheets-Sheet 1 NOV. 30, A, KRAVIS ETAL LOG PERIODIC ANTENNA WITH PLURAL CROSSED DIPOLES Filed April 29, 1960 2 Sheets-Sheet 2 INVENTORS Jaw/aw Y WWW By 5mm x 72 1;

AT TQRNEXS United States Fatent O 3,221,332 108 PERIODIC ANTENNA WITH PLURAL CRUSSED DIPOLES Alec Kravis, Maiden, and Matthew Frederick Redford,

Danhury, England, assignors to The Marconi Company Limited, a British company Filed Apr. 29, 1960, Ser. No. 25,622 Claims priority, application Great Britain, June 8, 1959, 1,515/59 5 Claims. (Cl. 343792.5)

This invention relates to aerial systems and has for its object to provide improved very high frequency (VHF) aerial systems of Wide bandwidth and high gain.

The principal applications of the invention are to VHF directional aerials, but as will be seen later, the invention may be used to provide a wide band VHF aerial system giving coverage in all horizontal directions.

According to this invention an aerial system includes at least one set of parallel dipoles spaced along and substantially perpendicular to the longitudinal axis of a twoconductor balanced feeder to which the halves of the dipoles are connected at their inner ends, corresponding halves being connected to one feeder conductor and the corresponding other halves being connected to the other, the dipoles being of different electrical lengths increasing substantially logarithmically from the connected end of the feeder to the other end and the conductors of the feeder being crossed over one another between adjacent dipoles, the spacings between which also increase substantially logarithmically from said connected end to the other end. Best results are obtained by choosing each spacing at a value between one half and one fifth of the electrical length of the shorter of the two dipoles spaced thereby, a preferred value of spacing being approximately one quarter of said electrical length.

Preferably the conductors of the feeder at the end where the longest dipole is situated are connected together by a terminating resistance. The said conductors may however be directly connected together at said end.

If there is only one set of parallel dipoles the aerial system will operate (if used for transmission) as a highly directional linearly polarised system radiating endwis'e, i.e. along the longitudinal axis of the two-conductor feeder, out from the end where the shortest dipole is situated. There may, however, be two sets of parallel dipoles connected to the same feeder conductors and transverse with respect to a common longitudinal axis, those of the second set being similar to, but at right angles to, those of the first with individual dipoles of the second set substantially midway between adjacent dipoles of the first. Such an aerial system, if used for transmission, will be a directional radiating system radiating endwis'e from the end where the shortest dipole is situated, but the radiation will be circularly polarised instead of linearly polarised.

In a further modification two component aerial systems, each having one set of parallel dipoles connected to a two conductor feeder as above described, are employed in combination and so arranged with respect to one another that the aerials of one set are at right angles to those of the other and transverse with respect to a longitudinal axis common to both feeders, individual aerials of one set being substantially mid-way between adjacent aerials of the other.

All the embodiments so far described are highly directional. Another feature of the invention, however, provides an omni-directional system of all-round coverage in one plane and high gain in the orthogonal plane. Ac cording to this feature of the invention an aerial system includes a plurality of dipoles spaced along and connected at their inner ends to the conductors of a two conductor balanced feeder, corresponding halves being connected to Patented Nov. 30, 1965 one conductor and the other halves being connected to the other, alternate dipoles being parallel to one another and at right angles to the other alternate dipoles and all being transverse to the same longitudinal axis, the said dipoles being of different electrical lengths increasing substantially logarithmically from the connected end of the feeder to the other end and spaced by spacings which also increase logarithmically from the said connected end to the other end, and the conductors of the feeder being spiralled round to cross over one another between alternate dipoles. As before, for best results the spacings should be in each case between one half and one fifth (preferably one quarter) of the electrical length of the shorter of the two dipoles spaced thereby. With this arrangement radiation is substantially at right angles to the longitudinal axis and, if the aerial system is mounted vertically (as it normally will be) with the shortest dipole at the bottom, there will be all-round coverage with horizontally polarised waves of approximately equal strength in all horizontal directions with limited radiation in the vertical plane while fluctuations due to ground reflections will be avoided to a great extent.

The invention is illustrated in the accompanying drawings which show diagrammatically a number of embodiments schematically.

Referring to FIG. 1 which shows a plane polarised highly directional aerial system in accordance with the invention, the said aerial system consists of an array of centre fed dipoles and an associated feeder. The dipoles are indicated by the references D1, D1, D2, D2, D3, D3 and so on, and they are connected to points along a two conductor balanced feeder F. The dipoles which are all parallel to one another and at right angles to the general direction of longitudinal extension of the feeder are of graded electrical lengths and graded spacings, the grading being logarithmic. In other words, the dipoles increase logarithmically in length from the shortest dipole (D7, D7 in FIGURE 1) at one end to the longest (D1, D1) at the other, and the spacings between adjacent dipoles similarly increase logarithmically from one end to the other, the spacing between the dipoles D6, D6 and D7, D7 being approximately equal to one quarter of the length of the dipole D7, D7, the longest spacing between any two dipolest-hat between D1, D1 and D2, D2being similarly approximately equal to one quarter of the length of the dipole D2, D2. The choice of spacings is not critical but best results are obtained by choosing for each spacing, a value between one half and one fifth of the electrical length of the shorter of the two dipoles separated by the spacing concerned. In FIGURE 1 the dipoles are shown as unloaded and in consequence their physical lengths and their electrical lengths are the same, but, as will be well understood, if the dipoles are loaded, their physical lengths will be less than their electrical lengths.

The two halves of any One dipole are connected each to one or other of the two different conductors of the feeder F and as will be seen the feeder conductors cross over between each pair of adjacent dipoles. Connection is made through a two-wire connector C at the shortest dipole end of the feeder F to a so-called balun represented merely by the rectangle B, connection from the balun being made to a transmitter or receiver, as the case may be, through a coaxial line X. It is, of course, not essential to use a balun, for a coaxial lead may be used for one of the conductors of the balanced feed, with its inner conductor connected to the other conductor at the far end of the aerial system. Preferably, and as shown, the conductors of the feeder F are connected together at the far end, i.e. the longest dipole end, by a terminating resistance R. The aerial system of FIGURE 1, when used as a transmitter, will radiate linearly polarised waves with high directivity in the longitudinal direction of the arrow in what may be called the backward direction i.e. along the extension of the axis in the direction D1, D1, to D7, D7 as indicated by the arrow A. The aerial system has a really wide bandwidth; indeed, a bandwidth of 3 to 1 has been obtained experimentally.

It will be seen that the proportions of the array are so chosen that the spacings of successive dipoles are approximately equal to their half lengths. The working portion of the array at any given frequency within its hand will consist of these dipoles which are at or near half wave resonance. As these are spaced approximately a quarter of a wavelength apart and fed in anti-phase, the aerial radiation will be backwards. Dipoles of less than resonance length present high impedance to the feeder and therefore take little energy from it. Dipoles at or near resonance present low impedance and are excited by the feeder. Relatively little energy is left in the feeder beyond the resonant portion of the array. Dipoles which are not resonant contribute to the radiation in much the same way as do the passive reflectors and directors of a conventional Yagi array and thus increase the aerial gain.

Theoretically the frequency range of the aerial is limited only by the lengths of the longest and shortest dipoles and, as already stated, a practical frequency range of 3:1 has already been obtained experimentally. The number of dipoles which are active radiatorsv is governed by the rate of increase of dimensions along the array, the more gradual the increase the higher the gain. The feeder spacings must be chosen to give an impedance high enough to ensure the wasting of little power in the load resistance R, but low enough to ensure that all the dipoles at or near resonance are energised.

FIGURE 2 shows part of a modification of the aerial system of FIGURE 1, only a few of the dipoles being shown. The difference between FIGURES 1 and 2 is that in the latter figure there are two sets of dipoles, those of each set being parallel to one another and those of one set being at right angles to and interspersed between those of the other, dipoles of one set being connected to the feeder about half-way between dipoles of the other. The dipoles of each set increase in length logarithmically as in the case of FIGURE 1 and the spacings of the dipoles of each set also increase logarithmically as in FIGURE 1. In FIGURE 2 the dipoles of the second set are distin guished from those of the first by the reference 2 in front of the dipole identification reference: thus 2D7, 2D7 are the two halves of the shortest dipole of the second set.

Each of the sets of aerials of the aerial system of FIGURE 2 will radiate directionally endwise as does that of FIG- URE l, but the combined radiation will, of course, be circularly polarised instead of linearly polarised.

FIGURE 3 shows a further modification which again consists of two sets of dipoles, each composed of parallel dipoles with the dipoles of one set perpendicular to and interspersed between the dipoles of the other. The lengths and spacings of the dipoles of each set increase logarithmically as before. However, whereas in FIG- URE 2 all the dipoles are connected to the same feeder, FIGURE 3 may be regarded as consisting of a combination of two systems each as in FIGURE 1 with the aerials interspersed: i.e. in FIGURE 3 there are two feeders F and 2F, one for one set of dipoles and the other for the other, each feeder having its own connector C or 2C at the shortest dipole end and its own balun B or 2B with associated coaxial cable X or 2X. In order to clarify FIGURE 3, the feeder 2F, the two-wire connector 2C, the balun 2B and the coaxial cable 2X are shown in broken lines to avoid confusion with the full line representation of the feeder F. The aerial system of FIGURE 3 is also an endwise directional radiator, but is consists of two complete component aerial systems (which may conveniently be mounted on a single mounting boom, not shown) each with its own separate feeder, one array will 4g transmit or receive vertically polarised waves while the other will transmit or receive horizontally polarised waves, assuming that one set of dipoles is vertical and the other horizontal.

FIGURE 4 shows a horizontally polarised aerial system in accordance with the invention and which radiates all round in the horizontal plane and has limited radiation in the vertical plane. This aerial system may be regarded as a modification of that of FIGURE 1, the modification consisting in effect in rotating each dipole through with respect to those on either side, the balanced twin feeder F being twisted into a spiral or helix to accommodate this. Resonant and near resonant elements (di oles) in the same plane, i.e. parallel to one another, are now substantially a half wavelength apart instead of substantially a quarter of a wavelength apart (as they are in FIGURE 1) and are thus fed in phase. The result is that practically all, the radiation is at right angles to the longitudinal axis of the array which will normally be mounted so that the said axis is vertical with the dipoles supported by a simple vertical mast carrying the shortest dipoles at the bottom. Since the dipoles lengths taper towards a vertex at the base of the mast level with the ground, the height of the resonant part of the array in wavelengths can be made practically independent of frequency thus to a large extent avoiding fluctuations due to ground reflections.

The arrangement of FIGURE 4 differs, of course, from that of FIGURE 2 in that, in FIGURE 4, adjacent dipoles are in a logarithmic relationship and are spaced apart by approximately a quarter of a wavelength while adjacent dipoles of the arrangement of FIGURE 2 are spaced by approximately an eighth of a wavelength.

The aerial system of FIGURE 4 will radiate horizontally polarised waves with substantially equal strengths in all horizontal directions. The vertical gain depends on the number of dipoles radiating at once and thus on the rate of change of length. In FIGURE 4, G represents the ground. The other references correspond to those of FIGURE 1 and therefore require no further description. A terminating resistance R is not shown in FIGURE 4, but it may be and preferably is provided at the top of the feeder.

FIGURE 5 is still another illustrative embodiment of this invention which is substantially identical to FIGURE 1 except that a direct connection has been made between the feeders at the ends of the feeders adjacent the largest dipole D and D1, which direct connection has been substituted for the resistance R.

Aerial systems in accordance with this invention can be used for transmission or for reception and stacked to give additional gain, and those of FIGURES 1 to 3 may obviously be employed in conjunction with parabolic or other reflectors for concentration or beaming of radiation.

We claim:

1. An omni-directional aerial system including a plurality of dipoles spaced along and connected at their inner ends to the conductors of a two-conductor balanced feeder, alternate dipoles being parallel to one another and at right angles to the other alternate dipoles and all being transverse to the same longitudinal axis, the said dipoles being of different electrical lengths increasing substantially logarithmically from the connected end of the feeder to the other end and spaced by spacings which also increase logarithmically from the said connected end to the other end, and the dipole feeder connections being spiralled round to cross over one another between alternate dipoles.

2. An aerial system as claimed in claim 1 wherein the spacings between adjacent dipoles is between one half and one fifth of the electrical length of the shorter of the two dipoles spaced thereby.

3. An aerial system as claimed in claim 2 wherein said spacing is substantially one quarter of the electrical length of the said shorter of the two dipoles.

4. An aerial system including a first set and a second set of parallel dipoles spaced along and substantially perpendicular to the longitudinal axis of a two-conductor balanced feeder to which the halves of the dipoles are connected at their inner ends, the dipoles of each set being of different electrical lengths increasing substantially logarithmically from the connected end of the feeder to its other end and the dipole feeder connections for each set being crossed over one another between adjacent dipoles thereof, the spacings between which also increase substantially logarithmically from said connected end to the other end, both sets of parallel dipoles being connected to the same feeder conductors and positioned transversely with respect to a common longitudinal axis, those of said second set being similar to, but at right angles to, those of said first set with individual dipoles of the second set substantially mid-way between adjacent dipoles of the first set.

5. An aerial system comprising in combination two component aerial systems, each component system including a set of parallel dipoles spaced along and substantially perpendicular to the longitudinal axis of a twoconductor balanced feeder to which the halves of the dipoles are connected at their inner ends, said dipoles being of different electrical lengths increasing substantially logarithmically from the connected end of the associated feeder to its other end and the dipole feeder connections being crossed over one another between adjacent dipoles, the spacings between which also increase substantially garithmically from said connected end to the other end, said component aerial systems being so arranged with respect to one another that the dipoles of one component system are at right angles to those of the other and transverse with respect to a longitudinal axis common to both feeders (individual dipoles of one component system being substantially mid-way between adjacent dipoles of the other component system.

References Cited by the Examiner OTHER REFERENCES Channel Master Corp. K.O. Antenna, copyright 1955; 3 pages.

Du Hamel and Ore: Logarithmically Periodic Antenna Designs, 1958 IRE National Convention Record, Part 1, March 24-27, 1958, pages 1-39151.

HERMAN KARL SAALBACH, Primary Examiner. GEORGE N. WESTBY, Examiner. 

1. AN OMNI-DIRECTIONAL AERIAL SYSTEM INCLUDING A PLURALITY OF DIPOLES SPACED ALONG AND CONNECTED AT THEIR INNER ENDS TO THE CONDUCTORS OF A TWO-CONDUCTOR BALANCED FEEDER, ALTERNATE DIPOLES BEING PARALLEL TO ONE ANOTHER AND AT RIGHT ANGLES TO THE OTHER ALTERNATE DIPOLES AND ALL BEING TRANSVERSE TO THE SAME LONGITUDINAL AXIS, THE SAID DIPOLES BEING OF DIFFERENT ELECTRICAL LENGTHS INCREASING SUBSTANTIALLY LOGARITHMETICALLY FROM THE CONNECTED END OF THE FEEDER TO THE OTHER END AND SPACED BY SPACINGS WHICH ALSO INCREASE LOGARITHMICALLY FROM THE SAID CONNECTED END TO THE OTHER END, AND THE DIPOLE FEEDER CONNECTIONS BEING SPIRALLED ROUND TO CROSS OVER ONE ANOTHER BETWEEN ALTERNATE DIPOLES. 